This invention relates to a method for removing multivalent heavy metals from metal plating waste effluents and, more particularly to, the use of an exfoliated vermiculite cation-exchange column for that purpose.
Metal plating is used to improve surface properties of metallic and non-metallic products, whereby an object is coated with one or more relatively thin, tightly adherent layers of some metal or metals. The metals commonly encountered in the plating process are nickel, copper, chromium, zinc, gold, silver, cadmium and tin.
Metal coating may be applied by several different processes. Electroplating is perhaps the most common and important plating operation with respect to both prevalency and water pollution control.
One of the two basic types of metal electroplating baths is the simple salt or "acid" both. For instance, in the copper plating industry the acid plating bath consists of a copper sulfate-sulfuric acid solution with two electrodes immersed in it. These electrodes are connected to a direct current electrical source; a copper plate serves as the anode and the metal surface to be plated as the cathode. Briefly, the copper sulfate dissociates, and as a current is applied, one electrode takes on a negative charge, the other a positive charge, thus causing copper ions to migrate toward the negative electrode (cathode). Upon reaching the cathode, two electrons are accepted by the copper ion, which then becomes a copper atom and attaches itself to the cathode. At the same time the free sulfate ion moves to the positive electrode (anode); the copper atoms of the anode give up two electrons reacting with the sulfate, thus forming more copper sulfate which dissociates and allows more copper ions to pass into solution.
This plating step is the source of divalent copper metal ions in the waste water. In Buford and Mosselli, Industrial Wastes, Reinhold Publishing Corp. (1953) at Chapter 13 entitled "Plating Wastes" the main sources of wastes from electroplating are listed as including: (a) drag-out losses carried into the overflow rinses by the plated element or the rack holding the plated element - drag-out losses can contaminate rinse waters with copper ranging from a trace to as much as fifty times that actually plated on the work; (b) solution dumping of spent, spoiled, or obsolete solutions (this is rarely done with metallic plating baths, unless remedial measures fail); and (c) tank leakage losses.
Waste water from a copper plating operation will rarely containing copper only. Quite often other metals are plated at the same plant and the waste stream will contain two or three or more heavy metals. However, just considering the copper portion, a typical rinse water from general plating operations may contain from 0.5 to 32 ppm copper. When this is taken along with the fact that wasteflow from plating plants may vary from 1000 gallons per day to over 310,000 gallons per day, it can be seen that large amounts of copper are being dumped in the nation's waters.
Trace quantities of metals are important constituents of waters, and are necessary for the growth of biological life. The presence of metals in excessive quantity, however, hinders aquatic life and could prohibit water usage from human consumption, because of the toxic effects of the metals.
For this reason, the U.S. Public Health Service drinking water standards contain restrictions concerning the maximum permissible amounts of chemical substances allowed in water supplies. A more detailed and comprehensive listing of the toxic effects of plating wastes can be found in an Environmental Protection Agency publication entitled "Water Quality Criteria Data Book", Volume 3, Effects of Chemicals on Aquatic Life (Pub. No. 18050 GNV 05/71).
Accordingly, it can be seen that there is a need for an effective means to remove these multivalent heavy metals from plating waste effluents to avoid polluting the water supplies. Of course, this is a recognized need and numerous waste water treatment methods have been proposed.
The methods suggested for treating metal plating wastes include: dilution and discharge into sewers or streams; evaporation, dialysis and electrodialysis; reverse osmosis, and land disposal. But the two most common treatment schemes for plating wastes are: (1) neutralization - precipitation methods and (2) the ion-exchange process.
In the neutralization-precipitation methods, the two most common bases used to precipitate heavy metals as their insoluble hydroxides are calcium hydroxide and sodium hydroxide. While calcium hydroxide is the cheaper of the two materials it is more difficult and costly to feed in small amounts, so in small acid neutralization systems sodium hydroxide, inspite of its cost, is usually the material of choice.
However, heavy metal flocs are difficult to settle as they have approximately the same density as water. Accordingly, to aid settling it may be necessary to add aluminum sulfate or some other coagulant. The settled sludge can be removed to either a sludge thickener, sludge drying bed, pressure or vacuum filter or a centrifugation dewatering process, but its disposal often presents further problems.
Ion exchange resins can also be used to concentrate the ions in plating waste to facilitate their removal or may be used to concentrate and recover them for reuse in the actual plating process. The most important advantage of the ion exchange treatment system is the water saving in the plant, up to 90%, owing to water recirculation. This, then, leads to a drastic waste volume reduction.
In Von Ammon, "New Developments in the Treatment of Metal Finishing Wastes by Ion Exchange of Rinse Waters", Purdue Univ. Ind. Waste Conf. Proc., 22:788 (1967) there is reported the use of ion exchange units to remove zinc, chromium, copper, nickel and silver with some being recovered for reuse, but most being removed by precipitation from the regeneration waste for disposal as a sludge. Other literature on use of ion exchange columns to remove heavy metals from plating wastes includes: Tallmage, "Ion Exchange Treatment of Mixed Electroplating Wastes", Ind. and Engr. Process Design and Devel., 6:4 (1967); McGarvey, "Brass and Copper Industry," Indus. and Engr. Chem., 44:534 (1952); Goddard "Ion Exchange Effluent Treatment and Wastes Product Recovery", Process Eng. (1975); Bloodgood, "Twenty Years of Industrial Waste Treatment," Purdue Univ. Ind. Waste Conf. Proc., 20:182 (1965); and "Waste Treatment", Upgrading Metal-Finishing Facilities to Reduce Pollution, EPA Technology Transfer Seminar Publication, 1973.
However, in Dean, "Removing Heavy Metals from Waste Water", Environmental Science and Technology, 6:518 (1972), it is indicated that the industrial response to this form of waste treatment has been hindered since certain waste stream impurities are destructive to resins, interfering ions are often present, there is a limited loading capacity, and there are relatively high operating costs. Further, a major problem is that even after removal, the resultant sludge is difficult to dispose of in an acceptable manner. Accordingly, the need still exists for an efficient, low cost method of removing multivalent heavy metals from plating wastes by use of cation-exchange columns while minimizing these problems.